Rick's Free Auto Repair Advice

Car AC hissing noise

Car AC hissing noise — What causes it?

A hissing noise from your car’s AC is perfectly normal. Car AC systems work by pressurizing liquid refrigerant and then releasing through a very small opening. Depending on the type of system used by the carmaker the metering device can be an orifice tube usually located in the engine compartment or an expansion valve often located in the cabin area. As they release the high-pressure refrigerant they make a slight AC hissing noise, similar to the noise you hear when you depress the nozzle on a spray can.

When does the AC hissing noise occur?

The AC hissing noise is most noticeable when you shut off the vehicle and you’ll hear it coming from under the dash area. That’s the sound of the high-pressure refrigerant flowing through the metering device and it will continue until the pressure is depleted and the system is “equalized” the same pressure on the high and low sides of the system.

Is the AC system cooling properly?

If the AC system is pumping out cold air and you hear AC hissing noise, don’t worry, everything is working as planned. However, if the AC system isn’t cooling properly and you hear a hissing sound, you may have an obstruction in the metering device. See this post for more information on what causes obstructions.

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Posted on by Rick Muscoplat

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